The present invention relates to a process for separating the elemental group III component of a group III-V material from aqueous polishing or etching wastes generated during the manufacture of group III-V material semiconductor devices.
Semiconductor devices formed from group III-V materials, such as, for example, gallium arsenide, gallium phosphide and indium phosphide, are used for a multitude of military and commercial devices in the United States and throughout the world. Typically, these uses include lasers, light-emitting diodes, and communications equipment. Manufacturing processes devoted to the fabrication of these devices generate large volumes of wastes which contain valuable gallium metal and indium metal. For example, gallium is particularly expensive and currently sells for about one dollar per gram. For that reason, low-cost procedures devoted to the recovery of these metals are economically advantageous to semiconductor manufacturers.
Manufacturers grow bulk crystals of group III-V materials in large boules or ingots. These boules are then cut into wafers, and etched and lapped to remove any surface damage. The wafers are then polished to achieve a mirror-like finish on one or both sides of the wafer. To polish the wafers, the wafers are mounted onto polishing plates, and either a wax or a vacuum is used to hold them in place. The polishing plates are then mounted on a polisher and are pressed against an abrasive polishing pad. The polishing is done "wet." In a "wet" polishing process, a very fine polishing agent, such as alumina, and an agent containing an oxidizing species are used to remove surface materials through a combination of mechanical and chemical action.
Use of the oxidizing species results in solubilized metal ions according to, for example, the following generalized reactions: EQU GaAs+"oxidizer".fwdarw.Ga.sup.+3 +As.sup.+5 +"residual oxidizer" EQU InP+"oxidizer".fwdarw.In.sup.+3 +P.sup.+5 +"residual oxidizer"
A number of chemical oxidizers have been used in the laboratory and in industry to polish group III-V semiconductor materials. In general, it is desirable to utilize chemical species which aid the polishing operation by oxidizing group V elements, such as arsenic or phosphorous, to the water-soluble +5 valence state, because the use of acids (without oxidizer) leads to the generation of toxic gases, such as arsine (AsH.sub.3) or phosphine (PH.sub.3). The most commonly used oxidizer species are hydrogen peroxide, chlorinated compounds (especially hypochlorite), and nitric acid. Typical concentrations of oxidizer, depending on the oxidizer species, of 10% to 30% are used during a polishing operation.
This wet polishing process produces an aqueous waste stream that contains from 200 to 400 ppm each of dissolved group III elements and group V elements, as well as residual oxidizer concentrations of from about 3% to about 10%. At such concentrations, the aqueous wastes from group III-V material semiconductor wafer polishing require subsequent treatment for removal of toxic materials, such as arsenic, prior to discharge of the aqueous waste from the manufacturing plant. The aqueous waste itself has a "milky" appearance, due to large concentrations of very fine polish particles, having sizes of 0.5 micron and smaller, suspended within it. While some of the suspended polish settles after time, most remains suspended in the aqueous polishing solution providing the milky appearance and complicating any separation processes. The pH of the aqueous waste is dependent upon the initial oxidizer solution used, but the resultant aqueous waste is generally more basic than the initial solution, due to a number of factors such as, for example, the presence of polish and generation of soluble group III element ions. Typically, these aqueous wastes have a pH near the neutral region.
Currently, aqueous wastes containing group III-V materials are treated with a soluble ferric iron species (e.g., ferric chloride or ferric nitrate) which is added to the aqueous waste. The pH of the aqueous waste is then adjusted so as to precipitate insoluble ferric hydroxide. The group III element ions and the group V element ions are "co-precipitated" with the ferric hydroxide. Coagulating and flocculating agents are added to aid in the physical removal of the resultant precipitate. This process produces a large volume of waste solids which must be disposed of, and which could readily leach toxic metals, such as arsenic, in a land-disposal environment. The colloidal nature of the polishing agent also complicates the physical separation process. For this reason, it is difficult to obtain consistent metal concentrations in the discharged filtrate on a day-to-day basis. Consistent metal concentrations in the aqueous waste are particularly important when the discharge contains toxic metals which are subject to environmental output regulations.
An additional concern with this conventional process is that the two materials for which recycling is desirable, the group III element and the group V element, are intimately mixed with a tremendous excess of a third material (iron). Therefore, recovery and recycling of the group III elements and the group V elements from the iron precipitate are extremely difficult. This, combined with the problems sometimes encountered with meeting toxic metal discharge limits for the filtrate, results in a need in the art for a method that not only recovers valuable components of group III-V materials, but does so in a way which treats for toxic components.